TryEngineering Hands-on Activity
Wireless Wonders: Code-Division Multiplexing
Explore the concept of code-division multiplexing, a key theory behind many wireless technologies. When we use a cell phone, our voice is converted into digital signals (zeros and ones). In this activity, student teams act as cell phones (all chatting simultaneously), while the teacher represents a cell tower. All transmissions happen at the same time and within the same frequency range, with messages overlapping. A special coding scheme allows the cell tower to decode all the messages correctly. Multiple phones transmit, and the cell tower decodes them all.
Topic Areas
- Wireless
Age Range of Students
- 11-13 years
- 14-18 years
Students will understand the technical concepts behind wireless technology, including:
- How different devices can efficiently share the same channel
- The principles of code-division multiplexing
- The role of transmissions and frequency in wireless communication
- Handout with Walsh Codes
- Welsh Code Tool with Instuctor Notes (Google Sheet)
- Student Transmission Google Sheet (instead of doing the transmission manually)
- Whiteboard at the front of the room
- Randomly assign each student (or group of students a code 1-8). You can do this by write each code on a piece of paper or use the created google sheet: Student Sheet: Code-Division Multiplexing (each team is assigned a different tab). Each code has 8 positions (8-chip long codes). In the real world no limit on the codes (8 is just arbitrary to simplify the exercise).The instructor should not know who has each code!
- Then they each make a decision of what “message” they want to transmit (0 or 1) but they don’t tell anyone.
- If they select one as their message, they write their walsh code (0->-1, 1-> 1).
- If they select zero as their message, then they write the inverse of their walsh code (0->1, 1->-1).
- Then the organizer adds up everyone’s answers for each chip in the sequence and writes the sum of all the answers on the whiteboard/smartboard/whatever (so for example, if 5 students had -1 for a chip and 3 students had 1, then they would write down -2).
- Finally, for the grand reveal, the organizer multiplies the results they just wrote down by each walsh code one chip at a time, and shows that the secret answers (0 or 1) selected by the students can be decoded again using only that combined signal and the known walsh code. (greater than zero ->1, less than zero -> -1)
- At the end, we can introduce some terms such as “auto-corrolation” and “cross-correlation”. You can also show that if one of the chips is damaged, that it would not impact the answer.
- This doesn’t require advanced math (it’s basic addition and multiplication) and so can be done with younger people.
- Transmissions are not in code order – can be in any row they want. Transmitted info from each team of students and all go into this sheet.
Thanks to the IEEE Communications Society (ComSoc) and it’s members who created this hands-on activity:
- Marc Blumberg
Lesson Plan Translation
